USPTO Patents Application 10762114

-Evm:5D3DE7mJs

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SYSTEM AND METHOD FOR PERFORMING SIMOX
IMPLANTS USING AN ION SHOWER

FIELD OF THE INVENTION

The present invention relates generally to ion shower systems, and more
particularly to a system and method for performing SIMOX type implantation using an
ion shower system.

BACKGROUND OF THE INVENTION

Silicon-on-insulator (SOI) technology offers particular advantages in the
fabrication of certain integrated circuit (IC) devices, as well as in other applications.
Among these advantages is higher performance over conventional devices, reduced
power consumption, improved radiation immunity, smaller die size, and process
simplification. Tools that facilitate the economical production of high quality starting
material, or wafers, to the SOI community can help drive the technology to greater
acceptance.

Several different techniques presently exist to form SOI type wafers. One
conventional process employs the implantation of hydrogen into the wafer to assist in
fracturing a wafer assembly comprising a wafer with a surface-deposited oxide layer
bonded to another silicon wafer. The implanted hydrogen preferentially allows the
assembly to fracture along a plane parallel to the oxide surface, resulting in a thin
surface silicon-on-oxide sandwich on a silicon substrate.

Another conventional technique employed to form SOI wafers is a technique
called "separation by implanted oxygen" (SIMOX). In the SIMOX process, a thin
layer (e.g., about 1,000-3,000 Angstroms) of a monocrystalline substrate is separated
from the bulk of the substrate by implanting oxygen ions into the wafer to form a
buried dielectric layer (BOX). Such implantation conventionally is performed with an

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implantation dose of about 2x1 0 17 to about 2x1 0 18 oxygen ions/cm 2 , and the resultant
buried dielectric layer ranged in thickness from about 1,000-5,000 Angstroms. The
SIMOX process thus results in a heterostructure in which a buried silicon dioxide
layer serves as an effective insulator for surface layer electronic devices.
5 Traditional SIMOX processing employs an ion implantation system, wherein a

pencil-shaped beam or a ribbon-shaped beam is generated, mass analyzed and
directed toward an end station. The end station is a batch-type end station, wherein
a plurality of workpieces or wafers reside and spin about an axis. In pencil-type
beams 10, wherein the beam width is substantially smaller than the size of the wafer

10 12, a magnetic scanner apparatus is employed to radially scan the beam with respect
to the endstation, such that as the wafers spin 14 about the axis, the oxygen ion
beam scans 16 across each of the wafers, as illustrated in prior art Fig. 1A. The
above solution requires a scanning mechanism and associated controller. In
addition, as can be seen in prior art Fig. 1B, however, such a scanning process is not

15 trivial; rather since some portions of the beam will be traversed twice per full scan,
while other portions are scanned only once, a moderately sophisticated scan and
rotation control architecture must be controlled to emulate a typically desirable
Lissajou pattern.

When employing a ribbon-shaped beam 20 as illustrated in Figs. 2A and 2B,
20 the width 22 of the beam is typically larger than the diameter 24 of the wafer 12, and
thus many of the above challenges associated with the above conventional scanning
process are avoided. Use of a ribbon-beam 20, however, has challenges with
respect to wafer cooling. Typically, a SIMOX process is controlled modestly
stringently at about 600C such that the implantation self-anneals to repair the lattice
25 of the wafer. Thus challenges exist to balance the beam power with radiative cooling
that is employed as the wafer spins about the axis. In addition, although the ribbon-
beam does not have to scan across the wafer, since the wafers are off-axis the

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current density seen by portions of the wafer further away from the axis decreases by
1/r, wherein r is the distance from the axis to the portion of the wafer at issue. Thus,
non-uniform implantation and thermal effects may occur unless additional control is
employed. Varying the current density of the ribbon-beam along its width to
5 accommodate for the above variation is rather challenging and requires further
system complexities.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basic
1 0 understanding of one or more aspects of the invention. This summary is not an

extensive overview of the invention, and is neither intended to identify key or critical
elements of the invention, nor to delineate the scope thereof. Rather, the primary
purpose of the summary is to present some concepts of the invention in a simplified
form as a prelude to the more detailed description that is presented later.

The present invention is directed to an ion shower system for use in
implantation of ions into a workpiece. The system is particularly advantageous for
use in SIMOX applications, wherein a high oxygen fraction and high beam current
uniformity is desired.

According to one aspect of the present invention, the ion shower system
comprises a plasma source associated with a chamber. The system further
comprises a workpiece support structure associated with a top portion of the
chamber, wherein the workpiece support structure is operable to secure a workpiece
such as a 300mm silicon wafer. The support structure secures the workpiece such
that an implantation surface thereof is oriented downward toward the extraction
assembly. The plasma source receives an input source gas (e.g., oxygen) and
generates a plasma using, for example, RF power. The source and/or chamber are
configured such that the generated plasma is azimuthally symmetric within the

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chamber. The extraction assembly is operable to extract the uniform plasma
vertically toward the secured workpiece for implantation thereof. The upside-down
design advantageously facilitates reduced contamination since any contaminants
suspended within the plasma fall away from the workpiece upon a deactivation of the
plasma. The configuration also advantageously facilitates use of evaporative cooling
of the workpiece by configuring an evaporative cooling structure on a top portion of
the workpiece support structure, wherein such cooling can be provided in a spatially
uniform manner across the workpiece.

In accordance with another aspect of the present invention, the ion shower
system comprises a chamber having at least two grounding rods therein. The
chamber walls are preferably coated with silicon to reduce contamination, however,
when oxygen is the source gas, the oxygen reacts with the silicon chamber sidewalls,
causing the walls to oxidize and become an insulator. Thus the grounding rods in the
chamber serve to collect excess electrons during ion extraction. The grounding rods
are silicon coated and while one is biased to a ground potential, the other is biased
negative such that ions in the chamber sputter off any oxide that has formed on the
grounding rod. The ground potential and negative potential are then switched back
and forth between the rods to maintain a path for excess electrons while preventing
the rods from unduly oxidizing.

According to still another aspect of the present invention, the ion shower
chamber comprises a radial confinement system operable to radially confine the
plasma within the chamber. The confinement system, in one example, comprises a
magnetic device that generates multi-cusp magnetic fields that serve to prevent the
plasma from reaching the chamber sidewalls. In one example, the magnetic device
comprises a plurality of independently drivable coils that permit the strength of
resulting multi-cusp magnetic fields to be individually controlled for tuning. The
confinement system aids in the generation and maintenance of azimuthally

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symmetric plasma within the chamber, which advantageously facilitates uniform
beam current at the workpiece.

According to yet another aspect of the present invention, the ion shower
system comprises an extraction assembly that comprises a multi-electrode
arrangement. A first electrode, a plasma electrode, is associated with the chamber
and is biased at the same potential as the plasma. The plasma electrode has a
plurality of extraction apertures within an extraction region, however, due to the high-
density plasma within the chamber, the plasma electrode may have a transparency of
as low as 10%. A second electrode, an extraction electrode, is biased negatively
with respect to the plasma electrode to form an electrostatic field therebetween and
extract positive ions from the chamber. The extraction electrode has a plurality of
extraction apertures associated therewith that are aligned substantially with respect
to the extraction apertures in the plasma electrode. The extraction electrode further
comprises a plurality of interstitial pumping apertures that serve to pump excess
neutral source gas therethrough and substantially reduce a pressure associated with
the extraction assembly. The reduced pressure improves system reliability by
reducing discharges within the chamber due to increased pressure.

The pattern of extraction electrodes may be spatially uniform or alternatively
may vary to provide compensation for any plasma non-uniformities. For example, if
the plasma density within the chamber decreases slightly azimuthally (about the
periphery of the chamber), the extraction aperture density can be increased
peripherally to provide compensation, thereby further improving beam current
uniformity at the workpiece.

According to still another aspect of the present invention, the ion shower
system comprises an RF antenna system for generating a plasma within the system
chamber. A neutral source gas, for example, oxygen, is injected into the chamber.
The RF antenna system produces RF electric fields that excite charged particles in

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the chamber that cause a plasma to be generated due to collisions of the accelerated
charged particles with the neutral source gas atoms. The antenna system comprises
a plurality of conductive loop antenna segments serially coupled together through
capacitors. The arrangement reduces an undesirable non-uniform capacitive field
component by a factor of N, wherein N is the number of conductive loop antenna
segments. The reduction in the capacitive field components advantageously
facilitates an improvement in plasma uniformity within the chamber. The plurality of
conductive loop segments preferably are configured in an azimuthally symmetric
arrangement such that, to the extent that any non-uniformity exists for each segment,
such non-uniformity is itself azimuthally symmetric within the chamber.

To the accomplishment of the foregoing and related ends, the following
description and annexed drawings set forth in detail certain illustrative aspects and
implementations of the invention. These are indicative of but a few of the various
ways in which the principles of the invention may be employed. Other aspects,
5 advantages and novel features of the invention will become apparent from the

following detailed description of the invention when considered in conjunction with
the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

10 Fig. 1A is a plan view diagram illustrating a convention endstation and a

scanning mechanism for implanting ions into a plurality of workpieces using a pencil-
type beam;

Fig. 1B is a plan view diagram illustrating in greater detail the conventional
scanning of a wafer of Fig. 1 A, wherein a scanning path results in portions receiving
1 5 differing doping concentrations;

Figs. 2A-2B are plan view diagrams illustrating a conventional endstation and
a scanning mechanism using a ribbon-type ion beam;

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Fig. 3 is a side elevation view of an ion shower system in accordance with one
or more aspects of the present invention;

Fig. 4 is a graph illustrating an oxygen fraction obtained using the ion shower
system of Fig. 3 in accordance with one aspect of the present invention;
5 Fig. 5 is a side elevation view of a portion of the ion shower system of Fig. 3,

illustrating in greater detail a chamber portion of various subsystems associated
therewith;

Fig. 6 is a sectional view illustrating a portion of the ion chamber having
plasma flow lines associated therewith;
10 Fig. 7 is a side elevation view of an extraction assembly in accordance with

one aspect of the present invention;

Figs. 8-9 are plan views of a plasma electrode and an extraction electrode,
wherein the plasma electrode has extraction apertures and the extraction electrode
has extraction apertures and interstitial pumping apertures associated therewith;
1 5 Figs 1 0-1 1 are fragmentary sectional views of extraction assemblies, wherein

the assembly of Fig. 10 has extraction apertures and the assembly of Fig. 1 1 has
both extraction apertures and interstitial pumping apertures associated therewith;

Fig. 12 is a plan view of an antenna assembly employed for generating a
plasma in the ion shower system of Fig. 3 in accordance with another aspect of the
20 present invention;

Fig. 13 is a side elevation view taken along line 13-13 of Fig. 12 illustrating the
antenna assembly of the present invention; and

Fig. 14 is an exemplary schematic diagram illustrating an antenna assembly
similar to that of Figs. 12 and 13, according to an aspect of the present invention.

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DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to the drawings
wherein like reference numerals are used to refer to like elements throughout. The
illustrations and following descriptions are exemplary in nature, and not limiting.
5 Thus, it will be appreciated that variants of the illustrated systems and methods and
other such implementations apart from those illustrated herein are deemed as falling
within the scope of the present invention and the appended claims. The present
invention pertains to an ion shower system for use in implantation of ions into a
workpiece. The present invention may find particular application in the implantation

10 of oxygen ions, for example, in a SIMOX type process to form SOI wafers for use in
semiconductor processing. Other applications, however, may be available and are
contemplated as falling within the scope of the present invention.

Turning now to the figures, Fig. 3 illustrates an ion shower system 100 in
accordance with one aspect of the present invention. The ion shower system 100

1 5 comprises a chamber 1 02 supported by a plurality of support structures 1 04. The

chamber 102 may reside within a cabinet enclosure 106, a portion 108 of which may
house a feed gas (not shown) such as oxygen, one or more power supplies (not
shown), as well as other components as needed.

The chamber 102 has a bottom portion 1 10, a top portion 112 and side

20 portions 1 14, respectively. In one example, the side portions 1 14 comprise a
cylinder, thereby making the chamber radially or azimuthally symmetric. An
extraction assembly 1 16 is associated with the top portion 112 of the chamber 102,
and couples a process chamber 118 containing a workpiece (not shown) therein.
The workpiece is secure within the process chamber 118 with a clamping system

25 120, such as an electrostatic clamp, however, other mechanisms may be employed
and are contemplated as falling within the scope of the present invention. As will be
discussed in greater detail infra, the process chamber 118 may further include a

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pump 122 for removal of neutral gas from the process chamber to maintain a desired
pressure environment.

The chamber 102 further comprises an inlet gas port 124, and in the present
example, the port is associated with the bottom 110 thereof. Also associated with the
5 bottom portion 110 of the chamber 102 is an antenna system 126 comprising multiple
single loop antennas coupled together. As will be discussed in greater detail below,
the antenna system 126 is coupled to an RF source (not shown) and provides RF
excitation for plasma generation within the chamber, wherein the excitation is
azimuthally symmetric. Such excitation facilitates plasma uniformity within the
10 chamber 102 that advantageously facilitates beam current uniformity at the
workpiece.

The chamber 102 further comprises a grounding system 128, for example, a
plurality of grounding rods that operate to collect electrons while ions are extracted
from the chamber in order to maintain plasma stability. Further, the chamber 102

15 further comprises a plasma confinement system 130 associated with at least the side
portions 1 14 of the chamber. In one example, the plasma confinement system 130
comprises a plurality of multi-cusp magnets that generate multi-cusp magnetic fields
that extend into the chamber. The multi-cusp magnetic fields operate to confine the
plasma radially and aid in further facilitating azimuthal plasma symmetry within the

20 chamber 102.

According to one aspect of the invention, the ion shower system 100 has an
upside-down design, wherein the extraction assembly 1 16 and workpiece are
associated with the top portion 112 of the chamber 102. In the above manner, the
generated ions within the chamber 102 are extracted vertically upwards via the

25 extraction assembly 116 and with the workpiece oriented facing down, the ions are
implanted therein. Such an arrangement advantageously provides a reduction in
contamination at the workpiece. For example, any contaminants suspended in the

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plasma will fall to the bottom portion 1 1 0 of the chamber 1 1 0 upon deactivation of the
plasma due to the influence of gravity. Thus upon deactivation, any contaminants fall
away from the workpiece as opposed to falling towards the workpiece in a
conventional system wherein the workpiece is situated below the chamber. Further,
5 the vertical orientation of the system 100 advantageously aids in beam uniformity at
the wafer. For example, one factor in achieving beam uniformity is the alignment and
focus of beamlets through the extraction assembly 1 16. The vertical orientation
highlighted above allows the extraction electrodes to also be vertically oriented, and
such orientation avoids or reduces cantilevering forces that may negatively impact

10 alignment when assembling or maintaining the system.

Another advantageous feature associated with the upside-down design is the
configuration facilitates use of an evaporative cooling unit (not shown) in association
with the clamping system 120. Due to the high beam current, cooling the workpiece
is an important feature. Evaporative cooling is more effective than mere convective

15 cooling, and with the upside-down design, an evaporative cooling system may reside
on top of the clamping system 120 (e.g., an electrostatic clamp). For example, water
may be employed as the cooling medium and overlay the entire workpiece portion of
the clamping system 120. As the water heats up and boils, steam escapes and
energy is dissipated uniformly about the workpiece. Such a mechanism would not

20 operate in the same uniform manner if the process chamber 118 was oriented to the
chamber 102 in another manner.

The chamber 102 is a large volume chamber, for example, having a diameter
of about 80 cm and a height of about 60 cm. Such a volume is desirable for oxygen
implantation of a workpiece such as a 300 mm (30 cm) semiconductor wafer. By

25 having a diameter substantially larger than the workpiece diameter, plasma uniformity
within the chamber will extend beyond the extent of the workpiece, thereby facilitating
a high beam uniformity thereat. In addition, a large chamber volume advantageously

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aids in achieving a large 0 + /0 2+ fraction, for example, greater than 98% when using
the ion shower system for a SIMOX type process.

It was found that various wall reactions occur within the chamber 102, for
example:
5 0 + -0,

0 2+ -* 0 2f

20^ 0 2 .

In particular, the wall recombination (20-* 0 2 ) is particularly undesirable since such
recombination negatively impacts (decreases) the 0+/0 2 + fraction. By having the

10 large volume chamber 102, the volume to surface ratio is large and thus although
surface recombination still occurs, such recombination is subsumed by the 0+ ions
generated within the volume. Operating the above configuration at about 15kW
power and about 0.5mTorr pressure, a plasma density of about 1x10 11 /cm 2 is
achieved with an 0 + /0 2+ ratio substantially greater than 98%, as illustrated in Fig. 4.

15 The ion shower system 100 of Fig. 3 is not mass analyzed, thus the present

invention contemplates coating the interior of the chamber 102 with silicon. Thus any
"contamination" is silicon, and since the SIMOX process is performed into silicon,
such contamination is permitted and is not a problem. The source interior, however,
being silicon, will oxidize and become Si0 2 after exposure to oxygen plasma. Si0 2 is

20 not a problem with respect to contamination in a SIMOX process, however, as the

interior surface becomes an electrical insulator, the surface cannot be employed as a
ground reference to the plasma. Consequently, as ions are extracted, no path exists
for excess electrons, and thus the plasma will tend to float negative and at some
point eventually prevent ion extraction toward the workpiece.

25 Therefore the ion shower system 100 of the present invention employs the

grounding system 128 employing a plurality of grounding rods within the chamber
102. In one example, two silicon coated grounding rods 128 are employed, wherein

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the silicon coating is doped to make the rods more conductive. Preferably, the rods
are doped with a P-type dopant such as Boron since semiconductor wafer starting
material is typically a lightly doped P-type substrate.

In a preferred aspect of the present invention, one of the rods 128 is grounded
and serves to collect excess electrons during ion extraction, while the second rod is
biased negatively. Since the grounding rods 128 will tend to oxidize due to the
presence of oxygen within the chamber 102, the negative bias will cause the biased
rod to be sputtered clean by the plasma, thereby removing any oxide build-up
thereon. Subsequently, the role of the rods is reversed, and the previously biased
rod is grounded while the previously grounded rod becomes negatively biased. In
the above manner the ground reference is maintained and such sputtering of the
rods does not result in unacceptable contamination at the workpiece.

In accordance with one exemplary aspect of the present invention, a square
wave voltage is applied to the two grounding rods 128, wherein the voltage to each is
approximately 180 degrees out of phase with one another. In the above manner, at
any instant, at least one of the grounding rods is grounded and acts to collect
electrons during ion extraction while the other, negatively biased rod is being
sputtered. Alternatively, a timing arrangement may vary, wherein at least one of the
rods is grounded during the extraction process and any such variation is
contemplated as falling within the scope of the present invention.

Turning now to Fig. 5, the chamber 102 is illustrated in greater detail. As
discussed above in conjunction with Fig. 3, the chamber 102 includes a plasma
confinement system 130 along at least the side portion(s) 1 14 thereof. The plasma
confinement system 130 is configured and adapted to provide containment of plasma
along the periphery of the chamber. In one example where the chamber 102 is a
cylinder, the system 130 operates to provide radial confinement along a vertical
height 140 thereof. Such radial confinement may operate to advantageously control

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radial plasma uniformity (while maintaining azimuthal uniformity) within the chamber
102. In one example, the confinement system 130 operates to provide multi-cusp
magnetic fields along the periphery or side portions 1 14 of the chamber 102. As may
be appreciated, the multi-cusp magnetic field lines 144 operate to provide a
5 substantial impedance to the plasma since the plasma tends to not cross the

magnetic field lines. Thus the multi-cusp magnetic field lines extending into the
chamber provide radial plasma containment and contribute to azimuthal plasma
uniformity therein.

Any type of magnetic device may be employed to generate such fields, for
10 example, an alternating pattern of permanent magnets having north and south poles
that each encircle the chamber. Alternatively, and more preferably, the plasma
confinement system 130 comprises a plurality of coils 142, wherein the coils are an
alternating pattern encircling the chamber 102 such that neighboring coils have
currents traveling in opposite directions (e.g., being oppositely wound), resulting in
15 multi-cusp magnetic fields 144. In such an example, the coils each may be

independently driven (drive circuit not shown), thereby advantageously providing
tunability in the multi-cusp fields to maximize radial plasma uniformity within the
chamber 102.

In addition, the plasma confinement system 130 may extend to the bottom
20 portion 110 of the chamber 102, as illustrated in Fig. 5. In such a manner, the multi-
cusp magnetic fields may extend up into the chamber interior from the bottom portion
for improved plasma confinement and uniformity, as exemplified by the field lines
146. In one example, the plasma confinement system 130 on the bottom portion 110
of the chamber 102 comprises a plurality of permanent magnets have north and
25 south poles associated therewith, wherein the magnets are arranged to facilitate

plasma uniformity azimuthally, and any such arrangement is contemplated as falling
within the scope of the present invention.

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Fig. 6 illustrates exemplary plasma confinement within the chamber 102
employing the exemplary arrangement of Fig. 5. Fig. 6 illustrates contour lines
representing the plasma within the chamber in cross section. Note that in Fig. 6, the
side portion 1 14 of the chamber 102 has the multi-cusp magnets 142 that provide
multi-cusp fields 144 in a region 150 extending radially into the chamber. The fields
144 generate a plasma boundary 152 that follows generally the multi-cusp magnetic
filed lines along the side 1 14 as well as the bottom portion 110 of the chamber. In
addition, the plasma contour is seen at an extraction plane 154 near the top portion
of the chamber, and Fig. 6 illustrates a substantially uniform plasma at the extraction
plane 154. Such plasma uniformity advantageously results in beam uniformity at the
workpiece (e.g., a beam current variation across a 300 mm wafer of less than 2%).

According to another aspect of the present invention, the extraction assembly
116 operates to extract oxygen ions from the chamber 102 and direct such ions
toward a workpiece for implantation thereof at a given energy level. One exemplary
extraction assembly of the present invention is illustrated in greater detail in Fig. 7,
and comprises a pentode type electrode arrangement, wherein five electrodes 202-
210 are aligned and spaced from one another via dielectric spacers 212. A first
electrode 202 is the plasma electrode and contacts and attaches to the top surface
1 12 of the chamber 102. The plasma electrode 202 is biased with respect to the
other electrodes 204-210, but floats with respect to the plasma (e.g., at 120 kV with
respect to the workpiece that is typically grounded) within the chamber 102. Each of
the electrodes 202-210 have an extraction region 214 through which a plurality of ion
beamlets pass, and the extraction region has a diameter or size 216 that is at least
as large as a diameter of the workpiece and is preferably larger in order to ensure
beam uniformity thereat.

The second electrode 204 comprises the extraction electrode and is biased at
a voltage less than that of the plasma electrode 202 (e.g., 105 kV). The negative

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relative potential with respect to the plasma creates an electrostatic field operable to
extract positive ions (e.g., oxygen ions) out of the plasma chamber 102. The third,
fourth and fifth electrodes 206, 208, and 210 comprise an auxiliary electrode, a
suppression electrode, and a ground electrode, respectively. The ground electrode
5 210 is biased at the same voltage as the workpiece (e.g., 0V), while the suppression
electrode 208 is biased at a voltage that is negative with respect to the ground
electrode (e.g., -20 kV). The suppression electrode 208 operates to prevent
electrons from a plasma local the workpiece (operating as an electron shower for
charge neutralization at the workpiece) from entering the extraction assembly 116.

10 The auxiliary electrode 206 operates to step down the voltage and is biased at an
interim voltage (e.g., 40 kV) between the extraction electrode 204 and the
suppression electrode 208, respectively.

The plasma electrode 202 has a plurality of extraction apertures associated
therewith, as illustrated in Fig. 8 and designated at reference numeral 220. Fig. 8 is

1 5 not drawn to scale, but rather is provided for purposes of illustration. The extraction
apertures 220 are relatively small (e.g., about 3-4 mm in diameter), whereas the
entire region diameter 214 is greater than 300 mm. The extraction aperture centers
are spaced apart from one another (e.g., about 2 cm center-to-center), in a uniform
manner, in one example, with spacing such that the electrode transparency is about

20 10%. Use of a high density plasma of about 1x10 11 /cm 3 in the plasma chamber 102
permits the low transparency and also allows the plasma electrode 202 to be cooled
via fluid ports/channels therethrough (not shown), if desired.

Alternatively, the extraction apertures may be spaced from one another in a
non-uniform manner, with such non-uniformity configured to provide compensation

25 for any non-uniformity in the plasma within the chamber 1 02. For example, if the
plasma within the chamber 102 tends to "fall off" azimuthally, wherein a plasma
density along an outer periphery region (that may correspond to the workpiece

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periphery) is about 5-10% less than a plasma density at the center, the density of
extraction apertures 220 may be made greater along an outer periphery of the
electrode 202 (and other electrodes). In the above manner, more ions are extracted
along the outer periphery, leading to increased ion beam uniformity spatially at the
5 workpiece. Any variation in extraction aperture configuration to compensate for

plasma non-uniformities may be employed and is contemplated as falling within the
scope of the present invention.

The extraction electrode 204 and other electrodes 206, 208 and 210 also have
extraction apertures associated therewith that are aligned with respect to the

10 extraction apertures 220 of the plasma electrode. In addition, the electrodes 204-210
have interstitial pumping apertures associated therewith, as illustrated in Fig. 9 and
designated at reference numeral 222. As will be further appreciated below, the
interstitial apertures 222 substantially improve extraction reliability by substantially
reducing the pressure within the extraction assembly 116, thereby preventing

1 5 discharges from undesirably shorting out the electrodes.

The inventors of the present invention identified that the high pressure within
the chamber 102 could provide some operational disadvantages. For example, for a
relatively high pressure in the chamber 102 (e.g., about 1 mTorr), a substantial
amount of charge exchange occurs due to the ion beamlets colliding with neutral gas.

20 Adding a pump in the workpiece chamber 118 operates to pump out the neutral gas
passing through the extraction assembly 1 1 6 but not help substantially to reduce the
pressure because transparency through an initial extraction assembly is low and gas
conduction to the electrode sides is found to be relatively low, as illustrated in Fig. 10.
In such an instance, the pressure differential between the chamber 102 and a region

25 230 between the electrodes 202' and 204' of Fig. 10 is relatively low (e.g., about 0.1
mTorr) which can disadvantageously result in discharges that can short out the
extraction assembly.

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The electrodes 204-210 of the present invention employ the interstitial
pumping apertures 222 in addition to the extraction apertures 220, as illustrated in
Figs. 9 and 1 1 , that substantially lower the pressure in the extraction assembly (e.g.,
1-2 orders of magnitude compared to the chamber pressure). Consequently, gas
"constriction" exists at the plasma electrode 202, but substantial conductance is
provided in the assembly 1 1 6 thereafter due to the pump 122 in the workpiece
chamber 118 removing the neutral gas passing through the interstitial pumping
apertures 222.

As illustrated in Fig. 1 1, the interstitial pumping apertures 222 are not aligned
with the extraction apertures 220. Thus, the oxygen beamlets 234 still are extracted
through the extraction apertures 220 without being impacted, while the interstitial
pumping apertures 222 readily allow a plurality of neutral gas conducting paths
through the assembly 116. Pumping of such neutral gas through the extraction
assembly 1 16 is important to keep the pressure within the plasma chamber 102 from
getting too high and creating discharges. Although the interstitial pumping apertures
222 in Fig. 1 1 are illustrated as being aligned with respect to one another,
alternatively, such apertures may be staggered with respect to one another and such
a variation is contemplated as falling within the scope of the present invention.

In addition, it should be noted that the extraction assembly can be configured
in a variety of ways to regulate an amount of ion beamlet overlap at the workpiece to
adjust an amount of beam uniformity thereat. For example, as illustrated in Fig. 1 1 , a
potential difference between the plasma electrode 202 and the extraction electrode
204 creates an electrostatic field that pushes electrons in the plasma chamber 102
away from the extraction apertures 220 in the plasma electrode 202, resulting in a
plasma sheath 232 that appears like an inverted meniscus. The magnitude of the
voltage difference dictates the magnitude of the electrostatic field, thereby impacting
a shape of the plasma sheath. The sheath 232 acts as a lens for the ion beamlets

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234, thereby affecting a focus of each of the beamlets. As illustrated in Fig. 1 1 , the
plasma sheath 232 impacts a focusing of the beamlets, thereby allowing the
extraction apertures 220 of each of the electrodes (although still aligned) to be
uniquely sized so as to pass the ion beamlets while concurrently blocking
contaminants.

The beamlets 234 exit the last electrode (the ground electrode 210), and due
to the plasma sheath optics, each of the beamlets is slightly diverging. The
workpiece (not shown) is spaced away from the ground electrode 210 a
predetermined distance such that the beamlets 234 overlap at the workpiece surface
for beam uniformity thereat. In SIMOX type processing, it is desirable to obtain an
oxygen ion beam uniformity of < 1 % variation at the workpiece. The inventors of the
present invention have determined that such uniformity may be obtained if at least
three (3) beamlets 234 overlap at the workpiece surface. That is, a predetermined
distance is provided (in accordance with the plasma sheath 234) such that an edge
of one beamlet touches a beamlet center at least two beamlets away.

Turning now to another aspect of the present invention, the antenna system
126 will be described in conjunction with figures 12-14. Fig. 12 illustrates a plan view
of the antenna system 126 taken from a position inside the plasma source chamber
102 of Fig. 3, while Fig. 13 is a side elevation view taken along line 13-13 of Fig. 12.
And lastly, Fig. 14 is a schematic diagram illustrating an effective electrical circuit of
the antenna system 126 of Figs. 12-13.

Referring to Figs. 12 and 13, the antenna system 126 comprises a base 302,
for example, associated with a bottom portion 1 10 of the chamber 102 upon which a
plurality of antenna conductors 304 reside. The antenna conductors 304 are serially
coupled together through capacitors 306 associated with the base 302, and two of
the conductors are coupled externally to an antenna drive circuit 308 (Fig. 14),
through an RF coupling mechanism 310. In addition, the base 302 has an aperture

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312 within a central portion thereof, wherein an inlet source gas feed 314 extends
therethrough to form a portion of the inlet gas feed port 124 of Fig. 3. The inlet
source gas fed 314 provides a neutral gas (e.g., oxygen) in the chamber 102 for
ionization thereof to form a plasma, as will be discussed further below.
5 In accordance with one aspect of the present invention, the antenna drive

circuit may comprise an integrated power oscillator RF source such as that described
in U.S. Patent 6,305,316 and assigned to the assignee of the present invention,
which is hereby incorporated by reference in its entirety. In such an example, the
integrated power oscillator employs characteristics of the RF segmented antenna

10 within the oscillator tank circuit. Such incorporation advantageously facilitates high-
speed ignition of the plasma that is important, in some cases, because when the
system is activated the ion beam is immediately striking the workpiece and no time
exists for a manual tuning of the tank circuit. The integrated power oscillator
advantageously automatically adapts to the change in the plasma impedance during

15 plasma ignition.

The antenna conductors 304 are arranged in an azimuthally symmetric fashion
about the base 302 as illustrated in order to generate an azimuthally symmetric
plasma within the chamber 102. Such uniformity of the plasma operates to provide
advantageously a uniform beam current at the workpiece. Although six conductors

20 304 are illustrated in Fig. 12, it should be understood that any number "N" of such
conductors may be employed and such variations are contemplated by the present
invention, wherein "N" is an integer greater than 1. The antenna system 126 of the
present invention generates a plasma via inductive coupling, and the arrangement
provides a substantial reduction in variation along the antenna elements compared to

25 conventional arrangements without a need for a faraday shield.

The RF antenna system 126 of the present invention operates in the following
manner. A time dependent current is generated in the conductors 304 via the

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antenna drive circuit 308. The time-varying current produces a magnetic field that
surrounds the conductive elements in accordance with Faraday's law. Because the
current is time-varying, the produced magnetic field is a time-varying field. In
accordance with Maxwell's equations, the time-varying magnetic field induces a time-
5 varying electric field normal thereto, wherein the time-varying electric field extends
along a direction of the conductors 304 and decays as the field extends away
therefrom. This time-varying electric field is referred to as the inductive electric field
component since it is induced from the time-varying magnetic field.

The time-varying inductive electric field accelerates charged particles such as

10 electrons near the antenna conductors 304. Further, the antenna elements are

configured such that the velocity of the accelerated charged particles is sufficient so
that the charged particles move through the region associated with a conductor in a
time that is short compared to the period (T) of the time-varying current.
Consequently, the charged particles see a substantially steady field as it travels

1 5 along the conductor 304. Therefore the time-varying electric field "heats" the charged
particles that then have sufficient energy to ionize the source gas atoms within the
chamber 102 upon collision therewith. The ionizing collisions operate to generate the
plasma and such plasma generation is substantially azimuthally symmetric in
accordance with the configuration of conductors 304 about the base 302.

20 In addition, the current generated and flowing through the conductors is

produced by an RF voltage (e.g., 800V peak to peak). Therefore the conductor
elements 304 have a voltage across each element that spatially varies along a length
of the element. Consequently, the varying charge distribution along an element 304
produces an electrostatic field that extends from the conductors outwardly and the

25 strength of the field varies spatially along the length of the element. This electrostatic
field component is referred to as the capacitive field component. Because this field is
not uniform, the contribution of this field component to plasma generation is non-
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uniform, and thus it is desirable for this electric field component to be reduced as
much as possible.

Some conventional antenna designs employ a faraday shield to block or
minimize the capacitive field component. In such a solution, the faraday shield is
5 placed between the conductors and the plasma, however, such a solution increases
circuit losses and is practically difficult to configure since the conductors in the
present system are immersed in the plasma. The antenna system 126 of the present
invention overcomes the disadvantages of the prior art and provides a structure that
substantially reduces the capacitive field contribution of the system without use of a

10 faraday shield, as will be further appreciated below.

The antenna design 126 of the present invention divides what conventionally
was a single conductor into "N" conductor segments 304 (e.g., N=6 as illustrated in
Fig. 12, or N=7 as illustrated in Fig. 14), wherein each conductor segment 304 is DC-
isolated from one another, but in RF-series via capacitors 306. Such an arrangement

1 5 reduces the peak capacitive electric field component by a factor of N. Referring to
Fig. 14, preferably the values of L (associated with the conductors 304), C (the
capacitors), and o> (27rf, wherein f is the frequency of the drive signal from the
antenna drive circuit 308) are selected so that the reactance of the capacitive
component is the same absolute value as the reactance of the inductive component

20 (i.e., coL = 1/a)C). In the above manner, a resonant circuit exists, and the peak

voltage across each element 304 does not exceed the applied voltage divided by "N".
Although a voltage drop does exist across each conductive segment 304, the
maximum voltage drop is reduced by "N", thereby reducing the undesirable
capacitive field component by "N". Also, although some variation still occurs along a

25 length of an individual component 304, it is N times smaller than a conventional
arrangement, and as illustrated in Fig. 12, such variation is itself azimuthally

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symmetric due to the arrangement of the conductors azimuthally.

The antenna system 126 thus operates to generate a plasma within the
chamber 102, wherein the generated plasma is azimuthally symmetric. The
azimuthal symmetry of the plasma advantageously helps to provide a spatially
5 uniform beam current at the workpiece.

Although the invention has been illustrated and described above with respect
to a certain aspects and implementations, it will be appreciated that equivalent
alterations and modifications will occur to others skilled in the art upon the reading
and understanding of this specification and the annexed drawings. In particular

10 regard to the various functions performed by the above described components

(assemblies, devices, circuits, systems, etc.), the terms (including a reference to a
"means") used to describe such components are intended to correspond, unless
otherwise indicated, to any component which performs the specified function of the
described component (i.e., that is functionally equivalent), even though not

1 5 structurally equivalent to the disclosed structure, which performs the function in the
herein illustrated exemplary implementations of the invention. In this regard, it will
also be recognized that the invention may include a computer-readable medium
having computer-executable instructions for performing the steps of the various
methods of the invention. In addition, while a particular feature of the invention may

20 have been disclosed with respect to only one of several implementations, such
feature may be combined with one or more other features of the other
implementations as may be desired and advantageous for any given or particular
application. In accordance with the present invention, the term "ribbon-like" should
be understood to include both a ribbon beam and a scanned pencil type beam.

25 Furthermore, to the extent that the terms "includes", "including", "has", "having",

"with" and variants thereof are used in either the detailed description or the claims,
these terms are intended to be inclusive in a manner similar to the term "comprising".

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Also, the term "exemplary" as utilized herein simply means example, rather than
finest performer.

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